THE IRON-BEARING MEMBER. 287 



until a g-reat depth was reached, for the waters entering the formation 

 would steadily work their way to a greater and greater depth along the 

 foot-wall quartzite until such depths were attained as to prevent its farther 

 penetration. 



Now, suppose erosion to gradually sweep away the rocks w^hich are 

 between the old surface of the country and the present surface. Begin- 

 ning at the base, the rocks of the formation at the present surface would be 

 more and more exposed to the action of percolating waters. These waters 

 would in turn affect the middle and finally the higher layers of the forma- 

 tion until its whole width was subject to the agencies of alteration. There 

 is, then, a gradual increase in the time that jicn'olating waters have acted 

 upon the various horizons of the formation in passing from south to north. 

 The difference in time to which the highest and lowest la vers have been 

 subjected to such action is at least the length of tinic tliat it has taken 

 erosion to remove the thickness of rock ])etween the nld surface of the 

 country and its pi-esent surface. Therefore tlic slower we l)elieve the 

 erosion to have l)een the greater the difference in time. 



Next, suppose that ei'osion has continued until the surface of the land 

 is at some intermediate point. In tracing the percolating waters in their 

 passage through the formatittn it is necessary to take into account tlie deflec- 

 tion to which they would be subject caused 1)V its la}'ers, bv the inipene- 

 ti'able character of the underlying slates, and In' intei'secting dikes. The 

 relative positions of the ore bodies, quartzites, and dikes have alread\- ])een 

 given. The water which fell upon the layers of the iron f'oniiatiou near its 

 V)ase would readily pass through the rock; it being here alreacK iiuich 

 altered and broken by the long action of water. However, in the most 

 broken cherts there is evidence in the somewhat irregular liands of ore that 

 the path of percolating waters has l>een influenced b\- tlie stratiform char- 

 acter of the formation. Passing through these ferruginous cherts, the water 

 Mould (piickly reach a dike or tlie fragmental ([uartzite, and would follow 

 this l)arrier, deflected to the north if upon the (juartziti' and to tlie south if 

 upon a dike, until it reached a trough made by the dike and cpuirtzite, along 

 w iiicli it would travel toward the east as it peiietrattd deeper. Such water 

 would be likely to contain oxygen in solution, and wnuld be capable, if it 



